The 2015 South India floods resulted from heavy rainfall generated by the annual northeast monsoon in November–December 2015. They affected the Coromandel Coast region of the South Indian states of Tamil Nadu and Andhra Pradesh. More than 500 people were killed and over 1.8 million (18 lakh) people were displaced. With estimates of damages and losses ranging from nearly ₹200 billion (US$2 billion) to over ₹1 trillion (US$11 billion), the floods were the costliest to have occurred in 2015, and were among the costliest natural disasters of the year.
Though the unusually heavy rainfall in southern India during the winter of 2015 has been attributed to the 2014–16 El Niño event, in July 2018 the Comptroller and Auditor General of India (CAG) categorised the flooding across Tamil Nadu as a "man-made disaster", and held the Government of Tamil Nadu responsible for the scale of the catastrophe, which the latter had termed a natural disaster.
Background
Geographical and meteorological explanation
From October to December each year, a very large area of south India, including Tamil Nadu, the coastal regions of the Andhra Pradesh and the union territory of Puducherry, receives up to 60 percent of its annual rainfall from the northeast monsoon (or winter monsoon). The northeast monsoon is the result of the annual gradual retreat of monsoonal rains from northeastern India. Unlike during the regular monsoon, rainfall during the northeast monsoon is sporadic, but typically far exceeds the amount produced by the regular monsoon by up to 90 percent. This excessive rainfall can be exacerbated by an El Niño of order of the magnitude which has since been evaluated to every year, such as 2015.
The coastal districts of Andhra Pradesh usually bear the brunt of heavy rains that occur during the northeast monsoon; with numerous river systems and wetlands, Puducherry and eastern Tamil Nadu are prone to flooding. The city of Chennai alone experienced five major floods between 1943 and 2005, with the 1943, 1978 and 2005 floods causing particularly severe damage. In addition, unplanned and often illegal urban development has led to many wetlands and natural sinks being built over; this, along with ageing civic infrastructure and poorly designed drainage systems, has resulted in an increased frequency of severe flooding.
Weather systems
On 8 November 2015, during the annual Cyclone season, a low-pressure area consolidated into a depression and slowly intensified into a deep depression before crossing the coast of Tamil Nadu near Puducherry the next day. Because of land interaction and high vertical wind shear, the system weakened into a well-marked low-pressure area over north Tamil Nadu on 10 November. The system brought very heavy rainfall over the coastal and the north interior districts of Tamil Nadu. On 13 November, Kanchipuram recorded 340 mm of rain and several low-lying areas were inundated and Vembakkam close to Kanchipuram recorded 470 mm. On 15 November, a well-marked low-pressure area moved northwards along the Tamil Nadu coast, dropping huge amounts of rainfall over coastal Tamil Nadu and Andhra Pradesh with 24‑hour totals peaking at 370 mm in Ponneri. Chennai International Airport recorded 266 mm of rainfall in 24 hours. On 28–29 November, another system developed and arrived over Tamil Nadu on 30 November, bringing additional rain and flooding. The system dropped 490 mm of rainfall at Tambaram in 24 hours starting 8:30 am on 1 December. Very heavy rains led to flooding across the entire stretch of coast from Chennai to Cuddalore. A study from IISc shows that on 1 December, clouds were stationary over Chennai. This study showed that the mountains of Eastern Ghats blocked clouds that came from the Bay of Bengal from moving further inland. The mechanism is referred to as 'upstream blocking'. Therefore, they were stationary over Chennai throughout the day. These clouds gave continuous rainfall over Chennai that caused massive flooding.
Floods in Tamil Nadu
Chennai
Between 9 and 10 November 2015, Neyveli received 483 mm (19.0 in) of rainfall; rains continued to lash Cuddalore, Chidambaram, Tiruvallur, Kanchipuram and Chennai. Several low-lying areas in Kanchipuram, including major thoroughfare Gandhi Road, were inundated as the city and its neighbourhood received a heavy precipitation of 340mm during the 24-hours that ended with 8.30 a.m. on 13 November 2015. The Manjalneer Kalvai, primarily a flood drain channel for the city, overflowed after the Netteri lake breached on the Kancheepuram-Vellore Road in the wee hours of Friday, flooding the entire stretch of Gandhi Road and Munusamy Mudaliyar Avenue and forcing the police to close for traffic the prime junction Moongil Mandapam where the Gandhi Road joins with Vallal Pachaiyappan Salai. Kanchipuram Collector R.Gajalakshmi told reporters later that a total of 7,294 persons were rescued from inundated areas and accommodated in 26 shelters opened by the civic body. Boats were used to rescue the people from inundated areas in Pillayarpalayam, the Collector said. And also Vembakkam close to Kanchipuram recorded 470mm of rain on 13 November 2015. Continuing rains led to low-lying parts of Chennai becoming inundated by 13 November, resulting in the evacuation of over 1000 people from their homes. The flooding in Chennai city was worsened by years of illegal development and inadequate levels of flood preparedness. Much of the city remained flooded on 17 November, though rainfall had largely ceased. Chennai received 1,049 mm (41.3 in) of rainfall in November, the highest recorded since November 1918 when 1,088 mm (42.8 in) in of rainfall was recorded. Kancheepuram district registered the heaviest rainfall—183% higher at 181.5 cm as against average rainfall of 64 cm in October–December period and Tiruvallur district recorded 146 cm compared to average 59 cm of rain. The flooding in Chennai city was described as the worst in a century. The continued rains led to schools and colleges remaining closed across Puducherry and Chennai, Kancheepuram and Tiruvallur districts in Tamil Nadu, and fishermen were warned against sailing because of high waters and rough seas.
Though rainfall from the earlier low-pressure system ended on 24 November, another system developed on 29 November, bringing additional rain and the India Meteorological Department predicted heavy rainfall over Tamil Nadu until the end of the week. On 1 December, heavy rains led to inundation in many areas of Chennai. By afternoon, power supplies were suspended to 60% of the city while several city hospitals stopped functioning. The same day, Chief Minister of Tamil Nadu Jayalalithaa announced that, because of the continued flooding and rains, half-yearly school examinations originally scheduled for 7 December would be postponed until the first week in January. For the first time since its founding in 1878, the major newspaper The Hindu did not publish a print edition on 2 December, as workers were unable to reach the press building. The Southern Railways cancelled major train services and Chennai International Airport was closed until 6 December.
Chennai was officially declared a disaster area on the evening of 2 December. At the MIOT Hospital, 14 patients died after power and oxygen supplies failed. With a letup in rainfall, floodwaters gradually began to recede in Chennai on 4 December, though 40 percent of the city's districts remained submerged and safe food and drinking water remained in short supply. Though relief efforts were well underway across most of the area by 3 December, the lack of any coordinated relief response in North Chennai forced thousands of its residents to evacuate on their own. As intermittent rains returned, thousands of displaced residents from Chennai, Kancheepuram and Tiruvallur districts attempted to flee the stricken region by bus or train and travel to their family homes. Chennai International Airport was partly reopened for cargo flights on 5 December, with passenger flights scheduled to resume from the following morning. By 6 December, rescue efforts had largely concluded and relief efforts intensified, with the Chennai Corporation beginning to disburse relief packages. Mobile, banking and power services were gradually restored; fuel and food supplies were reaching their destinations, the airport had fully reopened and rail services slowly resumed. Many city neighbourhoods, however, remained flooded with some lacking basic necessities due to the uncoordinated distribution of relief materials. With the city slowly beginning to recover, state and national health officials remained watchful against disease outbreaks, warning that conditions were right for epidemics of water-borne illnesses to occur. Chennai Corporation officials reported at least 57,000 homes in the city had suffered structural damage, mostly those of working class. State housing boards said they would conduct safety inspections of both public and residential buildings. After being closed for the past month, schools and colleges across the affected districts began to reopen from 14 December. Relief operations were largely wound up by 19 December.
South of Chennai, heavy rains and flooding persisted into the second week of December. In Kancheepuram district, Chengalpattu, kanchipuram, Nandivaram- Guduvanchery, Perungalathur, Tambaram, Mudichur and Anakaputhur were inundated in floodwaters up to 7 metres deep by 5 December, which washed away roads and severed rail links; 98 people from the district were reported to have died. During 4–5 December, parts of Villupuram and Tiruvarur districts received up to 10 centimetres of rain, while some towns in Cuddalore district saw up to nine centimetres. Flood alerts were broadcast to 12 villages in the neighbourhood of the Tirumurthy dam in Tirupur district on 7 December, as the dam was likely to reach capacity within two days; the residents of those villages were urged to evacuate. Due to rainfall in Tirunelveli district, all of its dams had reached or were approaching full capacity by 7 December, forcing local authorities to discharge thousands of cusecs of water from reservoirs and causing the Thamirabarani River to reach flood stage. Torrential rains inundated hundreds of acres of paddy fields in Thanjavur, Tiruvarur and Nagapattinam districts, and caused residential areas to flood by 8 December. Large parts of Thanjavur city were marooned by rising waters, while several houses collapsed under the brunt of rainfall in Kumbakonam and Veppathoor.
After Chennai district, Cuddalore district was among those most severely affected by the flooding. Six of the district's 13 blocks suffered extensive damage during the floods in November. The resumption of heavy rainfall from 1 December again inundated the Cuddalore municipality and the district, displacing tens of thousands of people. Rains continued through 9 December. Despite the state government and individuals sending rescue teams and tonnes of relief materials to the district, thousands of those affected continued to lack basic supplies due to inadequate distribution efforts; this resulted in several relief lorries being stopped and looted by survivors. Large swaths of Cuddalore city and the district remained inundated as of 10 December, with thousands of residents marooned by floodwaters and over 60,000 hectares of farmland inundated; over 30,000 people had been evacuated to relief camps.
The state government reported preliminary flood damages of ₹84.81 billion (US$906 million), and requested ₹20 billion (US$214 million) for immediate relief efforts. On 5 December, a senior state revenue official said the state's official estimates of flood-related losses in November alone exceeded ₹98 billion (US$1 billion); he added the catastrophic flooding thus far in December could escalate total losses to well over ₹1,000 billion (US$11 billion). On 9 December, Chief Minister Jayalalithaa wrote to Prime Minister Modi, requesting him to immediately declare the "unprecedented, catastrophic and cataclysmic" flooding as a "national calamity." Aon Benfield, a major UK-based reinsurance broker, classified the floods as the costliest to have occurred in 2015, and the eight-costliest natural disaster of the year.